The present invention relates to an apparatus and method for recording information in a recordable optical information medium in which information can be additionally written only once and for reproducing information from the optical information medium.
When information are recorded or reproduced from an optical information medium, a recording beam, a tracking beam, a focusing beam and a reproduction beam are utilized. These optical beams are emitted from the same light source.
Among conventional optical recording/reproducing apparatuses, there is known an apparatus that can record and reproduce information with reference to various types of optical information mediums. To enable this, the apparatus is provided with a number of light sources for emitting optical beams of different wavelengths. The apparatus selects an optical beam that has the wavelength corresponding to the type of optical information medium in use, and uses the selected optical beam for the recording and reproduction of information. Even when this type of apparatus records and reproduces information with reference to an optical information medium, the optical beams used by the apparatus are from the same light source.
In recent years, the information-oriented society makes further progress, and the information processing apparatuses that are in general use can process information far faster than before. In accordance with this trend, there is a demand for recording/reproducing apparatuses that can record and reproduce information at far higher speeds. However, in the case of an optical recording/reproducing apparatus which is of a write-once type, the recording speed of the recording unit is slower than that the reproduction speed of the reproducing unit. As can be seen from this, the recording speed is generally lower than the reproduction speed in the present circumstances.
When information are recorded in a recordable optical information medium, the optical beam used for recording is of high energy, and the recording layer of the recording medium corresponds to such a wavelength as provides a great light absorption coefficient. Accordingly, recording pits can be formed with a low level of outputs. The reason for this will be described in more detail.
Let us assume that the moving speed of a recording head is V (m/s), the width of the recording light beam is D (m), the irradiation time of the recording light beam is ΔT (s), the output of the recording light beam is P (J/s), and the absorption coefficient of the recording layer of a recording beam is A. In this case, the amount of energy absorbed during the irradiation time ΔT (s) is represented as P×A×ΔT, and the area in which the energy is absorbed during the irradiation time ΔT is represented as V×D×ΔT. Accordingly, the average surface density W (J/m□) of the energy absorbed in area S is as follows:             (              P        ×        A        ×        Δ        ⁢                                   ⁢        T            )        /          (              V        ×        D        ×        Δ        ⁢                                   ⁢        T            )        =            (              P        ×        A            )        /          (              V        ×        D            )      
Given that the value of D is constant, the average surface density of the energy absorbed in the recording layer is in proportion to the absorption coefficient and in inverse proportion to the moving speed of the recording head.
As can be seen from the above, higher recording density can be obtained by increasing the output of the recording optical beam or by applying a recording optical beam having such a waveform as provides high absorption coefficient.
When information are reproduced from a recordable optical information medium, the optical beam radiated to the medium must have such a wavelength as provides a high contrast between the beam reflected from the recording pits of the recording layer of the medium and the beam reflected from the areas other than the recording pits of the recording layer. The higher the contrast the optical beam provides, the higher will be the S/N ratio (signal-to-noise ratio) of a reproduction signal. The factors that should be considered to obtain a high contrast include the reflection factor of an optical beam, the polarization angle thereof, etc.
In general, optical beams of the same wavelength do not satisfy both the requirements of a recording beam and those of a reproducing beam.
Among commercially-available recordable mediums, there is a medium whose recording layer is made up of a transparent support member arranged on the light-incident side, a metallic reflecting film, and a recording layer located between the support member and the metallic reflecting member and containing an organic pigment. When a light beam is radiated onto the recording layer, the energy the light beam has when it has passed the support member thermally changes the nature of the organic pigment. Since recording pits are thereby formed, information can be written only once. As shown in FIG. 2, the recording layer of this type of recording medium shows greatly different reflection factors between the case where a light beam having a wavelength close to that of near infra-ray light is incident on the recording pits and the case where the same light beam is incident on the areas other than the recording pits. When information are reproduced from this write-once recordable medium, a light beam which has a wavelength close to that of infra-red light and which does not vary in intensity with time is radiated onto the medium, and the amount of light reflected from the medium is monitored so as to check the difference between the amount of light reflected from the recording pits and the light reflected from the areas other than the recording pits. By monitoring the amounts of light reflected from the medium in this manner, the length of time during which the reflected light is intense and the length of time during which it is feeble are measured. In principle, the reflection factor of the above medium is about 80% when near infra-red light having a wavelength of 800 nm or thereabouts is incident on the areas other than the recording pits (i.e., unrecorded areas, curve b), and is about 40% when the same infra-red light is incident on the recording pits (i.e., the recording areas curve a). (These values of reflection factors are based on the assumption that the recording layer is a flat and smooth base member.)
If a near infra-red light of the same wavelength as the reproducing beam described above is used for recording information in the unrecorded areas of the above recordable information medium, the absorption coefficient of the light beam is as low as 20% or so, as indicated by curve b in FIG. 3 (100%−[reflection factor]−[transmittance]) (the transmittance can be regarded 0% or so). With such a low absorption coefficient, the recording speed cannot be improved.
The write-once recordable information medium described above exhibits a small reflection factor for an optical beam having a wavelength which is within the wavelengths of visible light; in other words, it exhibits a great energy absorption coefficient for that optical beam. This means that, when used for the recording of information, the optical beam provides a high energy efficiency and consequently speeds up the recording speed. At the same time, however, the optical light does not produce a significant difference between the amount of light reflected from the recording pits and the amount of light reflected from the areas other than the recording pits. This being so, an optical beam having the same wavelength as the recording beam cannot be used as a reproducing beam.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 2-187937 discloses a technique wherein information is recorded in a recording layer by using a recording beam whose wavelength provides the recording layer with a high optical absorbing coefficient and wherein the recorded information is reproduced from the recording layer by using a reproducing beam whose wavelength provides the recording layer with a low optical absorbing coefficient.
Even when the recording/reproducing method disclosed in KOKAI Publication No. 2-187937 is used, however, the wavelengths of optical beams must be different between the recording mode and the reproducing mode. In other words, a plurality of light sources must be provided, and one of them must be selectively used between the recording mode and the reproducing mode.
The recording/reproducing method disclosed in KOKAI Publication No. 2-187937 (i.e., the method wherein signals are recorded in a recording layer by using a recording beam whose wavelength provides the recording layer with a high optical absorbing coefficient and wherein the recorded signals are reproduced from the recording layer by using a reproducing beam whose wavelength provides the recording layer with a low optical absorbing coefficient) ensures a high S/N ratio of the reproduced signals and yet enables information to be recorded at higher speed. Even this method, however, is restricted by the energy of a maximum output of a light source. In the case where an information medium comprises an a recording layer having such a characteristic as is shown in FIG. 2, and information are recorded in the unrecorded areas of that recording layer, an optical beam that provides a high optical absorbing coefficient does not necessarily improve the recording speed.
As illustrated in FIG. 2, even where the absorbing coefficient of near infra-red light having a wavelength of 800 nm or so is about 20% for unrecorded areas, this absorbing coefficient increases to about 70% after information are actually recorded since the nature of the organic pigment is changed by the information recording. This being so, in the case where a recording beam having a wavelength that provides a low absorbing coefficient is radiated onto an unrecorded area, the absorbing coefficient of the recording layer is low immediately after the irradiation of the recording beam. In other words, the efficiency with which the energy is used is low at the start of the irradiation of the recording beam. As more and more recording pits are formed, however, the absorbing coefficient increases, and the efficiency with which the energy is used gradually increases. However, in the case where a recording beam having a wavelength that provides an absorbing coefficient of 50% or so is radiated onto the same unrecorded area, the absorbing coefficient does not increase. Hence, the recording speed is not improved.
Although a semiconductor laser is generally employed as a light source of an optical information recording/reproducing apparatus, the wavelength of the optical beam emitted thereby varies in accordance with the ambient temperature. If information are recorded by use of an optical beam having such a wavelength as results in a great change in the absorbing coefficient of the recording layer of the information medium, it is inevitable that the recording sensitivity will change greatly. To be more specific, the temperature of the semiconductor laser element changes due to variations in the ambient temperature, and the wavelength of the optical beam emitted by the semiconductor laser element inevitably changes. Since, therefore, the absorbing coefficient of the recording layer of the optical information medium greatly changes, the recording sensitivity is greatly changed, resulting in marked fluctuations at the time of information reproduction.
As described above, the laser element for emitting an optical beam having a waveform suitable for recording and the laser element for emitting an optical beam having a waveform suitable for reproducing are provided independently of each other. Accordingly, the adjustment needed for positioning the laser elements is hard to make and is thus costly.
It is known in the art that the optical beams emitted from the laser elements used for recording and reproduction cannot be focused on the same position without properly adjusting the relative position between the two laser elements (light sources) or the ratio between the focal distances of the collimator lens and the object lens.
To improve the recording density, the diameter ω0 of the beam spot which a recording optical beam forms on the recording surface (i.e., the diameter of zero-order light) must be as small as possible. In general, the diameter ω0 is determined as follows:ω0=0.32λ/sin θwhere θ is the output angle of an optical beam output or emerging from the object lens. Let us assume that θ is 30° because this output angle is common. In this case, sin θ is 0.5, so that the diameter ω0 is nearly equal to 0.6λ. Even if θ is 90°, sin θ is 1, so that the diameter ω0 is equal to 0.32λ. In connection with this point, please refer to Formula (1-20) appearing on page 26 of “Optical Disk Technology”, (Kabushiki Kaisha) Radio Gijutsu Sha, Feb. 10, 1989.